In systems utilizing compressed gases, for example, refrigeration systems and pneumatic power systems, coalescer filter units are frequently used to remove oil from the working gas being circulated through the system. This oil becomes mixed with the high pressure gas during compression by positive displacement compressors such as, for example, rotary screw compressors, in which oil is used to effect a gas seal between moving compressor parts and is picked up by the gases as an aerosol during the compression process. In some cases, most of the mass flow leaving the compression stage may consist of this oil aerosol and, although the bulk of this material will precipitate out of the flow as it progresses downstream, a residual must be filtered from the high pressure flow before it continues downstream to be introduced, for example, into the evaporator unit of a refrigeration system or pneumatic activators of a pneumatic power system.
Typically, this oil is removed by a spin-on type coalescer filter unit located a short distance downstream of the compressor unit. These spin-on type filter units characteristically include a filter head and a cylindrical filter element suspended beneath the head with its axis generally vertical. The head includes a high pressure inlet chamber, a lower pressure outlet chamber, and a filter element mounting surface with a threaded attachment nipple. The filter element includes a cylindrical canister which has a closed bottom end and an attachment plate at a head end with a threaded orifice to engage the threads of the nipple and hold the head end of the canister snugly against the filter element mounting surface. Typically, a cylindrical filter cartridge is mounted concentrically within the cylindrical canister and, when the filter element is attached to the filter head, high pressure gas may flow from the high pressure chamber of the head through a passage provided in the attachment nipple and into a hollow center of the filter cartridge. The gas then flows through a porous wall of the filter cartridge into an annular space formed between the filter cartridge and the filter canister, where it can then flow through openings provided in the filter element base and the filter element mounting surface into the lower pressure outlet chamber of the head and on through the system. As the compressed gas flows from the core of the filter cartridge radially outward through the cartridge wall to the annular space, oil contaminants coalesce in the porous material of the filter cartridge wall. The coalesced oil drops and migrates to the exterior filter cartridge surface, due to the influence of the pressure gradient over the filter cartridge wall. Once on the cartridge surface, the coalesced drops of oil flow, under the influence of gravitational force, to the bottom of the filter element canister. A fitting is provided at the bottom of the canister to attach an oil drain line to remove the coalesced oil.
By arranging the axis of the cylindrical filter unit vertically, locating both the gas inlet and outlet at the upper, head end of the filter element, and drawing the coalesced oil from the closed bottom end of the canister, the coalesced oil is accumulated in an area of relatively low gas flow velocity. This minimizes the gas content of the gas flowing through the outlet. While this arrangement of the prior art coalescer filters is functionally effective, it necessitates the removal of an oil drain line from the bottom end of the filter element canister whenever the filter element is to be changed or serviced. Limitations are also imposed on equipment design by the need to provide working room for tools used to attach and free the drain line and for the drain line itself.